As used herein, the term “double layer forming fabric” refers to forming fabrics comprising two sets of yarns oriented in a first direction, one set located on the paper side and the other set located on the machine side of the fabric, and which are bound together by a single set of binder yarns oriented in a transverse direction and woven as pairs. The weave patterns of each of the paper and machine side surfaces, as determined by the overall fabric weave pattern, are either substantially the same or different. Further, as used herein, the term “transverse” refers to either the machine direction or the cross machine direction of the fabric.
The binder yarns in the fabrics of this invention can be either weft yarns pairs, similar to those described, for example, by Johnson in U.S. Pat. No. 4,815,499, Barrett in U.S. Pat. No. 5,544,678, or Seabrook et al. in U.S. Pat. No. 5,826,627, or they can be warp yarn pairs such as are described in published US patent applications numbers 2003/0217782 by Nagura et al or US 2004/0020621 by Heger et al., or any of U.S. Pat. Nos. 5,152,326 to Vohringer, 4,605,585 to Johansson, 4,501,303 to Osterberg, and 6,223,780 to Kaldenhoff.
In the double layer forming fabrics of the present invention, all of the yarns oriented in the transverse direction as defined above comprise pairs of binder yarns, and the paper side layer and the machine side layer are each woven to provide different, but related, weave patterns.
As used herein, the following terms have the following meanings:
The term “binder yarn” refers to a yarn which occupies a path in the paper side layer and which separately interlaces with a machine side layer yarn to occupy a path in the machine side layer. Either the warp yarns or the weft yarns in the fabric may be used as binder yarns. All of the yarns oriented in the transverse direction (as described above) in the fabrics of this invention are binder yarns.
The term “drainage area”, expressed as a percentage of the area of the fabric weave pattern repeat, refers to the proportion of that area not occupied by the yarns, both warp and weft, used in weaving the fabric at a given substantially planar location within the fabric substantially parallel to the paper side surface and to the machine side surface of the forming fabric.
The term “fibre support index” or “FSI” refers to a calculation made according to the method described by Beran and summarized in Danby, R. & Perrault, J., “Weaves of Papermaking Wires and Forming Fabrics”, Montreal, QC, 90th Annual PAPTAC Meeting, Jan. 26-28, 2004, page 2, and which provides a measure of the number of support points which are available to support the papermaking fibres on the paper side surface of a given fabric weave pattern.
The term “float” refers to that portion of a component yarn which passes over a group of other yarns in the fabric without interweaving or interlacing with them; the associated term “float length” refers to the length of a float, expressed as a number indicating the number of yarns passed over. A float length can be expressed in terms of numbers of paper side layer or machine side layer warp or weft yarns.
The term “frame” refers to the substantially rectangular drainage area defined by the longitudinal axis of four interwoven yarns in the paper side surface of the paper side layer of a forming fabric. The number of frames per unit area is identified by the associated terms “frames/in2” or “frames/cm2”.
The term “interlace” refers to a locus at which a yarn forms at least one knuckle with another yarn in the machine side layer.
The term “internal float” refers to that portion of a component yarn which passes between two sets of yarns; the associated term “internal float length” in relation to this invention refers to the length of an internal float, expressed as a number indicating the number of PS yarns passed under.
The term “interweave” refers to a locus at which a yarn forms at least one knuckle with another yarn in the paper side layer.
The term “machine direction”, or “MD” refers to a line parallel to the direction of travel of the forming fabric when in use on the papermaking machine. The term “cross machine direction” or “CD” refers to a direction substantially perpendicular to the machine direction within the plane of the fabric. In the fabrics of the present invention, either the first direction, or the transverse direction, may be parallel to the MD, depending on the construction of the fabric and whether the binder yarns are warp or weft yarns i.e. if the warp yarns are the binder yarns, the transverse direction is parallel to the MD. The fabrics of the present invention are generally flat woven and seamed so that the warp yarns are oriented in the MD when the fabric is in use.
The term “paper side layer” refers to the layer in the forming fabric onto which the stock is delivered from the head box slice. The term “machine side layer” refers to the layer in the forming fabric in contact with the support means in the papermaking machine. Thus each of these layers has a paper side surface (“PS”) and a machine side surface (“MS”). In the double layer fabrics of the invention, the machine side surface of the paper side layer is adjacent to the paper side surface of the machine side layer.
The term “segment” refers to a portion of the single path occupied by a specific binder yarn in one repeat of the overall weave pattern, and the associated term “segment length” refers to the length of a particular segment, and is expressed as the number of paper side layer yarns with which a member of a pair of binder yarns interweaves within the segment.
Forming fabrics are used in papermaking machines to retain and support the papermaking fibres in the stock, to allow water to drain from the stock so that an embryonic fibrous web may form and to convey that web to subsequent areas of the papermaking machine. Initially these fabrics were woven from metal wire, typically phosphor bronze or stainless steel; in recent times yarns created from thermoplastic resins have become the material of choice. Currently preferred resins include polyesters, polyamides and various polymer blends.
The simplest forming fabrics are woven as single layer structures. Although single layer fabrics are known and used, they have several well documented disadvantages when used in certain papermaking conditions.
To overcome these disadvantages, double layer forming fabrics have been developed which consist essentially of two layers: these are a paper side layer which provides the surface on which an incipient paper web is formed, and a machine side layer which provides the surface that is in contact with the static supporting surfaces of the paper making machine. As noted above, within the overall forming fabric weave pattern, either warp yarns or weft yarns can be used as binder yarns which serve to hold the layers of the double layer fabric together and may contribute to the structure of one of the layers. It then follows that although the layers are bound together by the weaving process into a single fabric with a single overall repeating weave pattern, each of the layers is often constructed quite differently in terms of yarn sizes, yarn cross sectional shapes, yarn count (in terms of numbers of yarns per unit length), yarn fill (expressed as a percentage of the amount of yarns and their size relative to the total space available to accommodate them) and the thermoplastic polymer used in the yarns. It then also follows that at least the water handling capabilities, the wear resistance capabilities, and the strength capabilities of each layer, when considered separately, are commonly quite different.
Modern forming fabrics are woven so as to provide a paper side layer which imparts, amongst other things, a minimum of fabric mark to, and provides adequate drainage of liquid from, the incipient paper web. The paper side layer should also provide maximum support for the fibres and other papermaking solids in the paper slurry. The machine side layer should be tough and durable, and provide a measure of dimensional stability to the forming fabric so as to minimize fabric stretching and narrowing, or other distortions.
Weave patterns are known for double layer forming fabrics in which the warp yarns comprise pairs, alternately forming part of the paper side and the machine side weaves. In such patterns, when one member of a pair passes from the paper side layer to the machine side layer, the second member of the pair passes from the machine side layer to the paper side layer, thus completing the weave pattern and while binding the two layers together. Examples of such patterns are found, for example, in published US application Nos. 2003/0217782 of Nagura et al., 2004/0020621 of Heger et al., and in US patents 5,152,326 to Vohringer, 4,605,585 to Johansson, 4,501,303 to Osterberg, and 6,223,780 to Kaldenhoff. Others are known.
Nagura et al. in US 2004/0020621 disclose a double layer fabric in which warp pairs serve as binder yarns to interconnect the paper and machine side layer weft yarns. The yarns are arranged in the overall fabric pattern such that each warp yarn pair member replaces the other to complete the weave pattern of the opposite surface as the yarns exchange locations between the surfaces.
Heger et al. in US 2004/0020621 disclose a double layer forming fabric likewise comprised of pairs of warp yarns which alternately interweave with the PS and exchange positions to interlace with the MS and thus complete the weave pattern repeat of each of these two surfaces. The path taken by the two warp yarns as they enter into and exit from the PS is each different, and two warp of adjacent pairs must pass together under a common MS weft yarn.
Each of Vohringer, in U.S. Pat. No. 5,152,326, Johansson in U.S. Pat. No. 4,605,585 and Osterberg in U.S. Pat. No. 4,501,303 discloses a forming fabric having first and second yarn systems interconnected by a third system comprising pairs of yarns which together form a regular pattern on the paper side surface of the fabric. Kaldenhoff discloses a similar construction but uses both warp and weft binder yarns.
One problem which is common to all papermaking machines and which can have an adverse effect on the formation properties of the web, and has not been significantly addressed by these known weave patterns, is the problem of “impingement drainage” as will be discussed in greater detail below.
In the initial portion of either a single fabric or a twin fabric forming section (either with or without an initial open surface portion), an unsupported jet of highly aqueous stock is ejected at high speed from the head box slice onto the open surface of a moving forming fabric, or into the more or less convergent wedge shaped space between two moving forming fabrics. The jet of aqueous stock will typically traverse a short distance before impinging the surface of the forming fabric, or fabrics, at the point of impingement. The angle of impingement formed between the linear axis of the stock jet and the surface of the forming fabric, or fabrics, on which paper is made is generally quite small, and typically is of the order of from about 40 to about 100. Since the angle of impingement cannot be zero, which is to say tangential to the fabric surface, or fabric surfaces in a twin fabric paper making machine, at least in part because the stock jet widens in the direction perpendicular to the fabric surface or surfaces in the space between the head box slice and the point of impingement, the pressure exerted by the stock jet onto the forming fabric or fabrics can be resolved into two components: a component essentially tangential to the fabric surface, and a component essentially perpendicular to the fabric surface, both of which when combined have a considerable effect on impingement drainage rates. These forces are directly proportional to the speed at which the forming fabric moves in the machine direction: as the machine speed increases so do the impingement forces.
In modern high speed papermaking machines in which the forming fabric(s) can be moving at speeds up to 100 kph, the minor pressure component vertical to the fabric surface exerts a significant level of force on the forming fabric, which can cause excessive impingement derived drainage of the stock over the initial portion of the forming section. This minor pressure component (the “impingement pressure”) and the turbulent forces created by stationary drainage elements, combined with the increased use of particulate fillers and shorter papermaking fibres, have the effect of reducing first pass retention and increasing the embedment of the initial layers of the embryonic web into the paper side surface of the forming fabric.
It is well known that, on any papermaking machine under start up conditions and delivering a normal papermaking volume of water but without papermaking fibres from the headbox slice onto the forming fabric, this water will drain within a very short distance, approximately 12 inches (30 cm), or less than 1% of the total available drainage length of the typical forming section. This indicates that, without fibres, all forming fabrics have far in excess of the drainage capacity required to make paper. However, as soon as papermaking fibres are introduced, drainage is retarded at a rate determined by the length of the fibres, the quantity of fibres, the support characteristics of the papermaking surface of the forming fabric, and by the forces resisting and retarding impingement drainage. It was for this reason that the original forming boards installed on open surface fourdrinier type papermaking machines were so successful. In more modern twin wire formers such as gap formers, the impingement shoe serves that function.
It is known (from Johnson, Dale B., “Effect of Jet Impingement in Bel Baie Machines”, Pulp and Paper Canada, 9385 (1992)) that impingement drainage can cause sheet marking, low retention by the forming fabric of papermaking fibres, fines and fillers (i.e. low first pass retention), and plugging (i.e. sheet sealing) of the paper side layer of the forming fabric. Unless the structure of the forming fabric is designed to allow it to better manage and control impingement drainage, further increases in machine speed and/or paper making machine efficiency may be limited, or tied directly to improvements in forming shoe or forming board construction.
None of the prior art discloses fabric constructions that are specifically designed to retard the initial impingement drainage by means of long internal warp or weft floats which restrict the drainage area of the centre plane of the fabric. Although improved paper side layer surface uniformity can be obtained by using weave patterns in which warp yarn pairs together interweave with paper side layer weft yarns in sequence in a manner which provides a single unbroken warp path, known weave patterns have not addressed the problem of impingement drainage. Specifically, there has not been any teaching of weave patterns which allow the binder yarns to reside for greater lengths within the fabric as an internal float so as to close up this centre plane. Nor does the prior art address issues relating to fabric stability and wear resistance arising from a fabric construction which is designed to retard the initial impingement drainage. Both of these features are addressed in the present invention in ways which offer unexpected advantages as discussed herein.
It has now been found that the problems of impingement drainage can be significantly reduced, and the respective advantages of the preferred weave patterns for the paper side layer of these fabrics and the preferred weave patterns for the machine side layer can be retained, by the use of weave patterns in which the fabric layers are bound together by pairs of binder yarns, where the members of each pair together form a single path in the paper side surface, and alternately interlace with the yarns of the machine side layer to contribute to but not form a complete repeat of the machine side layer weave pattern; where the patterns further provide for long internal floats of the binder yarns between the paper side layer and the machine side layer, between successive interweaving and interlacing points. Such weave patterns increase the resistance of the central plane of the fabric to the impingement drainage forces and thereby provide numerous unexpected benefits, as will be discussed below.
Preferably the binder yarns are warp yarns, but they may also be weft yarns, depending on other physical attributes required for the weave pattern, based on the intended end use of the fabric.
It has been found that such weave patterns enhance the following fabric features in a manner not previously available in the prior art.
Firstly, the paper side surface of the fabric offers good sheet support with reduced sheet marking, yet provides sufficient drainage area to remove water to the interior of the fabric without entrapping fibres. This reduces fibre plugging or stapling, and so-called “sheet sealing” which makes removal of the embryonic web from the fabric difficult.
Secondly, the retardation of drainage in the area of the long internal floats of the binder yarns promotes good sheet formation and fines retention on the paper side surface of the fabric, with many of the same benefits to the sheet as are provided by the known forming boards and forming shoes.
Thirdly, the open drainage area of the paper side layer allows for easy passage of air through the sheet top surface to the paper side surface of and thereafter through the forming fabric as the fabric and sheet together pass over the suction boxes and similar drainage devices in the forming section. This high air passage over the vacuum zones will result in the sheet leaving the forming zone in a dryer condition, which will translate into greater efficiencies in both the press and drying sections of the paper machine.